Lipocortins (also termed "annexins") are a family of widely distributed calcium-binding proteins which have been studied in diverse biological systems, mainly with regard to their interactions with phospholipids and membranes, their potential role in membrane aggregation and regulation of cell growth and differentiation (Blackwood et al. (1990) Biochem J. 266:195-200; Burgoyne et al. (1989) Cell Calcium 10:1-10; Klee (1988) Biochem. 27:6645-6653). The lipocortins have also been implicated in inhibition of phospholipase A.sub.2 (PLA.sub.2) activity (Flower (1988) Br. J. Pharmacol. 94:987-1015). Phospholipase A.sub.2 is responsible for the release of arachidonic acid from membrane phospholipids. Arachidonic acid is a precursor in the synthesis of a number of compounds (e.g., prostaglandins, hydroxy-acids and leukotines) involved in the inflammation process. Accordingly, lipocortins are believed to inhibit inflammation by inhibiting the synthesis of inflammation-inducing substances.
Certain members of the lipocortin family have been shown to be generated in response to anti-inflammatory agents such as glucocorticoids (Smillie et al. (1990) Br. J. Pharmacol. 97:425P; Goulding et al. (1990) Lancet 335:1416-1418). A multiplicity of functions have been ascribed to the various annexins, including mediation of some of the anti-inflammatory effects of glucocorticoids in animal models (Cirino et al. (1989) Proc. Natl. Acad. Sci. (USA) 86:3428-3432; Errasfa et al. (1989) Br. J. Pharmacol. 97:1051-1058), involvement in signal transduction (Blay et al. (1989) Biochem J. 259:577-583; Creutz et al. (1987) J. Biol Chem. 262:1860-1868), calcium channel activities (Diaz-Munoz et al (1990) J. Biol. Chem. 265:15894-15899; Majima et al. (1990) J. Immunol. 145:1694-1699), exocytotic mechanisms (Ali et al. (1989) Nature 340:313-315), and blood anti-coagulant properties (Tait et al. (1989) J. Biol. Chem. 264:7944-7949; Funakoshi et al. (1987) Biochem. 26:5572-5578).
At least eight distinct proteins are now known to constitute this family of molecules in humans, characterized by a common core structure consisting of four repeats of a 70 amino-acid unit, which probably forms the calcium and phospholipid binding site (Schlaepfer et al. (1987) J. Biol. Chem. 262:6931-6937; Haigler et al. (1989) TIBS 14:48-51; Pepinsky et al. (1988) J. Biol. Chem. 263:10799-10811). However, questions concerning the significance of the differing N-terminal regions of the various lipocortins, the mechanism by which they exert their effects, and the physiological relevance of these proteins, remain unanswered.
The gene for human lipocortin-1 having a molecular weight of 37,000 daltons (37 kD) was the first to be cloned from the annexin family (Wallner et al. (1986) Nature 320:77-81), and its amino-acid sequence shares more than 50% homology with other members of the family (Pepinsky et al., ibid.). Inducibility of lipocortin-1 by glucocorticoids, and its inhibition of PLA.sub.2 activity have been demonstrated (Flower, ibid.: Goulding, ibid.; Hirata et al. (1980) Biochem. 77:2533-2536; Rothhut et al. (1989) Biochem. J. 263:929-935; Browning et al. (1990) in Cytokines and Lioocortins-1 in Inflammation and Differentiation (Melli et al., eds.) Wiley Liss, New York), but the physiological relevance of these findings as well as the mechanism by which lipocortin-1 inhibits PLA.sub.2 remain complex and unclear.
Recently it has been demonstrated that purified lipocortin-1 has an acute anti-inflammatory effect in the rat paw edema test (Cirino et al. (1989) Proc. Natl. Acad. Sci. (USA) 86:3428-3432), and mimics many anti-inflammatory effects of glucocorticoids including inhibition of neutrophil (polymorphonuclear cells, PMNs) superoxide generation (Maridonneau-Parini et al. (1989) J. Clin. Invest. 83:1936-1940), chemotaxis (Fradin et al. (1988) Biochim. Biophys. Acta 963:248-257), and production of eicosanoids, PGE.sub.2 and LTB.sub.4 (Glenney et al. (1988) Biochem. 27:2069-2076). A unifying characteristic of the diverse properties of lipocortin-1 is that its extracellular biological activities are dependent upon the presence, correct amino acid sequence and conformational orientation of the N-terminal domain (Browning et al., ibid.).